FIG. 1 shows a system for converting a received frame-relay frame into an ATM cell using a device performing a conventional direct memory access (DMA).
A processor 105 performs a process through a processor bus 106 according to an instruction from software. When receiving a frame-relay frame through a channel, a frame receiver 101 in a frame transmission processing device 102 searches for (hunts) a frame buffer. A data FIFO block 103 temporarily stores frame data. A processor bus interface 104 transmits the frame to the searched frame buffer in a memory 107 through the DMA.
A segmentation and reassembly (SAR) device 108 receives frame cell conversion information according to an instruction from the processor, and performs segmentation of the frame into an ATM cell.
The memory 107 is accessed by the processor 105 and the frame transmission processing device 102, and stores the frame cell conversion information 109 for conversion of the frame into an ATM cell and data 110 of the frame in the frame buffer as shown in FIG. 2.
The operation of the system will be explained. The processor 105 prepares an available frame buffer area in the frame transmission processing device 102. When starting to receive the frame receiver 101, the frame receiver 101 searched for the frame buffer and notifies the processor bus interface 104 of the top address of the frame buffer. The processor bus interface 104 transmits the received frame from the data FIFO block 103 to the memory 107 through the DMA according to the received address. At that time, the transmitted frame is stored in the frame transmission processing device 102. After completion of the transmission, an interrupt notification is sent to the processor 105.
By the software detecting the interrupt, the transmitted frame information is read from the frame transmission processing device 102. A multiprotocol section for the frame-relay frame which is written in the memory 107 is converted into a multiprotocol for ATM Adaptation Layer 5 (AAL5). While an area of an identifier for making a packet (capsuling) and transmitting the multiprotocol is assigned at the top of user data in the frame-relay frame and at the top of the AAL5 frame, the size of the area of the AAL5 frame is greater than the other kinds of protocols. To perform conversion between such protocols, the frame-relay frame is copied into a free area, which is not occupied by the frame data 11, in the memory 107 by the software, and the conversion of the protocols is performed. After completion of the conversion, the processor transfers the frame cell conversion information to the SAR device 108, which converts the frame into the AAL5 frame and produces an ATM cell.
However, there is the problem that the frame transmission processing device 102 may write the frame data from the top of the searched frame buffer. When the area for the multiprotocol of the AAL5 frame is greater than that of the frame-relay frame, the frame-relay frame must be copied into a free area, which is not occupied by the frame data 11, in the memory 107 by software for performing conversion between such protocols, increasing the overhead caused by the conversion process.
This is because the conventional frame transmission processing device 102 is not designed according to concepts in which the frame is lengthened due to the conversion of the transmitted frame data and in which the top of the frame is converted.